The use of plastic housings for electronic equipment and components is widespread in the automotive and electronic equipment fields as well as in other areas. However, plastic materials generally suffer from the disadvantage of being transparent or permeable to Electromagnetic Interference, commonly known as and referred to herein as xe2x80x9cEMIxe2x80x9d. This disadvantage is of considerable concern in view of the susceptibility of electronic equipment to the adverse effects of EMI, and to the large and growing number of consumer products which produce EMI, as well as the increasing regulatory controls exercised over such electromagnetic pollution.
Present enclosure design approaches are reaching their electromagnetic interference shielding limits. The need to contain the extraordinary high-frequency EMI found in today""s electronic equipment, coupled with the higher sensitivity of the equipment to EMI, has made enclosure design a more demanding task. For example, due to their smaller geometry, devices operating at higher frequencies are much more sensitive to EMI.
Plastic materials are highly desirable as materials of construction for cabinets used to house electronic equipment. They can be readily shaped and styled to make space saving, attractive enclosures. However, plastics are extremely poor electrical conductors and provide no EMI shielding unless highly modified.
One method of modifying the plastic is to paint its surface with a paint formulated with conductive material. However, such paints are labor intensive due to masking requirements and are prone to chipping and peeling and do not contribute to conductivity in the bulk of the plastic.
Alternatively, conductive filler such as carbon fiber or metal particles may be compounded into the plastic to be molded. These additives are expensive when used in sufficient quantity to achieve the desired volume resistivity (ohm-meter) necessary for EMI applications. In addition, the physical robustness of the structural plastic is significantly degraded at higher levels of conductive filler loading. Another EMI problem is potential loss of shielding effectiveness. This occurs as a consequence of RF leakage into or out of an enclosure along the interfacing surfaces between its outer low surface resistance film and its high volume resistivity inner plastic structure. The same effect can occur along the interfacing surfaces between the various conductive filler loaded molded parts of a multi-part EMI enclosure. The RF leakage is a consequence of the meniscus wetting characteristics of plastic that creates a thin insulating film along interface surfaces.
Accordingly, it is a principal object of the present invention to overcome the foregoing disadvantages and to provide a molded plastic article which has improved EMI shielding properties, e.g., no loss of physical robustness and enhanced RF shielding.
It is a further object of the present invention to provide a molded plastic article as aforesaid which is inexpensive and which may be simply, conveniently and expeditiously prepared.
Further objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention, the foregoing objects and advantages are readily obtained.
The molded plastic article of the present invention comprises: a plastic layer; an outer film layer over the plastic layer and forming a composite laminate therewith, the outer film layer having an inner surface adjacent to and bonded to the plastic layer, and an outer surface opposed to the inner surface; wherein the plastic layer and the outer film layer are both electrically conductive and reflect and/or adsorb electromagnetic interference. Preferably, the outer film layer will have lower surface resistance when compared to the plastic layer. In the preferred embodiment, the volume resistivity of the plastic layer ranges from about 3xc3x97106 to about 3xc3x97102 ohm-meter and the surface resistance of the outer film layer will be significantly less than the plastic layer and in the range from 101 to 10xe2x88x923 ohms/square.
The conductivity of the plastic layer is provided by incorporating either conductive fibers or conductive particles, and the outer film layer may also include conductive fibers or conductive particles, and may in addition include at least one of the following conductive materials: metal foil, metal mesh, carbon fiber, metallized film, metallized fiber and woven conductive fabric.
The molded plastic article of the present invention may include a third layer bonded to the plastic layer on the side thereof opposed to the outer film layer, wherein the third layer is desirably electrically conductive.
Further features of the present invention will appear hereinbelow.